The present invention relates to a semiconductor laser module for emitting laser light, and more specifically, to a semiconductor laser module for exciting an Er3+, Al3+ doped fiber amplifier (EDFA).
Conventionally, the wavelength of light emitted from a light emitting device is stabilized by using means that optically feeds back light from a multi-mode oscillation laser by any means, thereby making the oscillation wavelength mode of the laser single.
Lasers of this type include, for example, a distributed feedback (DFB) laser, in which a diffraction grating is formed on an active layer of a semiconductor laser device, a distributed Bragg reflector (DBR) laser, in which a reflective diffraction grating having a transparent reflection characteristic for emitted light with a wavelength different from the Bragg wavelength is formed along the longitudinal direction of a waveguide portion that is formed of a semiconductor medium of a semiconductor laser device, whereby the light is reflected on (or fed back to) an active layer, etc.
With the recent rapid progress of a fiber Bragg grating (FBG), in which an optical fiber is given a light diffraction function by changing the refractive index in a core in the axial direction, various techniques for lasers with FBG have been disclosed. For example, xe2x80x9cMulti-wavelength Bragg fiber-grating semiconductor laser arrayxe2x80x9d is described in the Technical Report of IEICE (the Institute of Electronics, Information and Communication Engineers) OPE 97-1 by Kato et al., and xe2x80x9cUV written waveguide grating and its application to integrated external-cavity laserxe2x80x9d is described in OPE97-2 by Tanaka et al.
For a light source for EDFA excitation, moreover, a technique for stabilizing the wavelength of a 1,480 nm band pump laser by means of a fiber Bragg grating (FBG) is reported in 2nd Optoelectronics and Communications Conference (OECC ""97) Technical Digest, July 1997, Seoul, Korea by Atsushi Hamakawa et al. (Draft Collection 9D2-5, pp. 224-225, xe2x80x9c1,480 nm pump fiber-grating external-cavity laser with two fiber gratingsxe2x80x9d).
The DFB and DBR lasers described in the aforementioned documents, however, have a single-mode oscillation spectrum. They are used for a dedicated light source for optical communication. Therefore, these lasers are not suited for EDFA amplification.
The external-cavity lasers described in the Technical Report by Kato et al. and Tanaka et al. have the following problems.
(1) The aforesaid external-cavity lasers are single-mode oscillation lasers, in which mode hopping is caused such that the center wavelength of the oscillation mode changes when the working temperature changes by several degrees xc2x0 C., so that the stability of the oscillation wavelength to resist temperature change is poor.
(2) The aforesaid external-cavity lasers are hard to manufacture, since the reflectance of an anti-reflection film on its front end face should be lowered extremely. In many cases, moreover, the external-cavity lasers are constructed so that laser beams are emitted at an angle to the cleavage plane of the laser in order to lower the reflectance of the anti-reflection film equivalently. According to the external-cavity lasers constructed in this manner, therefore, the coupling efficiency of the laser beams coupled to an optical fiber is lowered, and it is very hard to fix the optical fiber with center alignment.
(3) The aforesaid external-cavity lasers are designed so that the space between the emission surface of a laser device that constitutes an external cavity and a diffraction grating for optical feedback is narrow. Accordingly, assembling a module requires complicated processes, and therefore, use of special means such as plane mounting.
(4) Output power of the aforesaid external-cavity lasers is low. Therefore, they are unfit for use as light sources for EDFA excitation.
Although the technique reported by Atsushi Hamakawa et al. is suited for the excitation of a high-power EDFA of 1,480 nm wavelength that uses an InGaAsP/InP-based semiconductor film, it has the following problems.
(1) The aforesaid technique requires use of a special fiber Bragg grating that reflects two wavelengths.
(2) According to the aforesaid technique, the variation of the oscillation wavelength attributable to change of the working temperature, although small, is about 2.6 nm.
(3) The aforesaid technique cannot be applied to a GaAs/AlGaAs-based semiconductor laser with the same construction. The InGaAsP/InP semiconductor laser is not subject to degradation that is attributable to catastrophic optical damage (COD), and the mode field pattern of its emitted light is circular. On the other hand, the GaAs/AlGaAs-based semiconductor laser is subject to the COD-induced degradation and its mode field pattern is elliptic, so that the efficiency of mode coupling with an optical fiber based on a two-lens system is very poor.
(4) In a GaAs/AlGaAs-based multi-mode oscillation semiconductor laser having random wavelength characteristics without gain ripples, the oscillation wavelength can be stabilized by modularization such that the fiber Bragg grating (FBG) is coupled to the laser. In this semiconductor laser, the state of polarization of the optical fiber continually changes depending on the bent state of the externally attached FBG, stress acting thereon, distortion, etc. In a module that uses this semiconductor laser, therefore, a light touch on the optical fiber inevitably changes the value of the monitor output current by a figure, although the optical output and the monitor output current are apparently stable. Thus, the operating characteristics of the semiconductor laser module of this type considerably changes depending on the state of polarization, fiber shape, etc. of the optical fiber.
(5) In the semiconductor laser module coupled with the fiber Bragg grating, furthermore, mode competition between light oscillated in a mode of the FBG and light oscillated in the Fabry Perot (FP) mode causes fluctuation of the optical output power on the front end face of the semiconductor laser, which is attributable to mode hopping, and fluctuation of the optical output power on the rear end face that is used to monitor the optical output.
In this case, in particular, power fluctuation on the monitor side of the rear end face of the semiconductor laser (optical output on the rear end face side is converted into current by means of a photodiode in the semiconductor laser module) or fluctuation of the monitor output current is more susceptible than that on the side of the front end face of the semiconductor laser.
The semiconductor laser uses the monitor output current to control the optical output automatically. If mode hopping or mode competition occurs in the semiconductor laser, in this case, the monitor output current changes rectangularly or in the shape of spikes with time, so that automatic control is impossible. If the monitor output current of the semiconductor laser is reduced rectangularly, moreover, high current flows into the semiconductor laser in order to maintain the constant optical output, possibly damaging the semiconductor laser and arousing a great problem on the operational reliability.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a semiconductor laser module, which enjoys high oscillation wavelength stability against operating current injected into a semiconductor laser and temperature change, and is suited for use as a light source for EDFA excitation or a high-output, low-noise light source.
A semiconductor laser module according to the present invention, which includes the following means, can restrict fluctuation of optical outputs or fluctuation of monitor output current, which is caused in case of mode hopping or mode competition, within a practically negligible range, in consideration of the relation between a reflection center wavelength xcexBG of a Bragg grating and a gain peak wavelength xcexLD(I) for an operating current injected into a semiconductor laser, that is, an injection current (I).
In a pump laser for an optical fiber amplifier, a GaAs/AlGaAs-based semiconductor laser having a resonance mode form of a gain wavelength characteristic in a natural emission region shown in FIG. 1A and a net gain form shown in FIG. 1B is designed to construct a semiconductor laser module of an external-cavity type by using optical feedback means such as a Bragg grating. In this case, the module has the following features based on the relation between the reflection center wavelength xcexBG of the Bragg grating and the gain peak wavelength of the semiconductor laser.
The following is a description of the meanings of a maximum operating current Iop and a pulling wavelength width xcexPULL.
Normally, the maximum operating current Iop is a maximum injection current that ensures oscillation of the semiconductor laser. If the injection current exceeds a certain value, the optical output is saturated, so that there is no linear relation between the optical output and the injection current. This is a kink phenomenon as it is called. If a high current is injected to cause the kink phenomenon, the refractive index of each component layer that constitutes the semiconductor laser is lowered by a plasma effect, band-filling effect, etc. Accordingly, the semiconductor laser cannot confine light that is generated in an active layer, so that the optical output emission angle changes, and the optical output changes substantially. The injection current that causes the kink phenomenon is referred to generally as a kink current. Thus, in the semiconductor laser module, the maximum operating current Iop of the semiconductor laser is generally adjusted to a current value that is equivalent to a value as low as 15 to 20% of the output power of the kink current, in consideration of the reliability of the module (see dotted line of FIG. 12).
The output characteristic of the optical output (mW) for an injection current I (mA) will be further described in detail with reference to the characteristic curves of FIG. 12. If the optical output that causes the kink phenomenon and 20%-lower optical output in FIG. 12 are Pkink and Pop, respectively, the operating current value corresponding to the optical output Pop is Iop. In FIG. 12, the full line represents the initial characteristic of the semiconductor laser before aging, and the injection current value corresponding to the optical output Pop is IBOL (beginning of life). On the other hand, aging occurs when the semiconductor laser starts to be used. In order to maintain a constant output despite the advance of degradation, the semiconductor laser is expected to ensure that the operating current never exceeds a given level. The maximum value of the injection current for this case is given by IEOL (end of life). Thereupon, the maximum operating current Iop according to the present invention must be set within the range of xcex94Iop indicated by the arrow in FIG. 12.
On the other hand, the pulling wavelength width xcexPULL is the difference between the reflection center wavelength xcexBG of the Bragg grating and the gain peak wavelength with which the oscillation mode of the semiconductor laser module changes from the Bragg grating mode into the Fabry-Perot mode. For example, the value of the difference (=xcexBGxe2x88x92xcexFP) between the reflection center wavelength xcexBG of the Bragg grating in the semiconductor laser module and a gain peak wavelength xcexFP that is obtained when the semiconductor laser undergoes Fabry-Perot oscillation is equal to the pulling wavelength width xcexPULL. In the present specification, in particular, the gain peak wavelength xcexFP that is obtained when the semiconductor laser undergoes Fabry-Perot oscillation is referred to as a shorter-wavelength-side limit value xcexLIMIT of the pulling wavelength width xcexPULL for the duration of the Bragg grating mode oscillation.
In order to stabilize the optical output, narrow-band oscillation wavelength (e.g., 5 nm or less), and monitor output current characteristic of the semiconductor laser, a semiconductor laser module according to the present invention is a semiconductor laser module of an external-cavity type, in which a GaAs/AlGaAs-based semiconductor laser having ripples in the gain-wavelength characteristic thereof and an optical transmission medium including a Bragg grating are optically coupled by means of optical coupling means, designed so that a gain peak wavelength xcex(Ith) for the case where the modularized semiconductor laser is driven with a threshold current Ith is set on the shorter-wavelength side of the reflection center wavelength xcexBG of the Bragg grating, a gain peak wavelength xcex(Iop) of the semiconductor laser for a maximum operating current Iop is set on the longer-wavelength side of the gain peak wavelength xcex(Ith), a pulling wavelength width xcexPULL and a de-tuning width xcexdetun of the semiconductor laser module defined below are set so that the de-tuning width xcexdetun is smaller than the pulling wavelength width xcexPULL, the resulting difference (=xcexPULLxe2x88x92xcexdetun) is greater than the full width at half maximum of the reflection spectrum of the Bragg grating, and the gain peak wavelength xcex(Ith) is on the longer-wavelength side of a limit value xcexLIMIT described below.
The pulling wavelength width xcexPULL of the semiconductor laser module is the difference between the reflection center wavelength xcexBG of the Bragg grating and a gain peak wavelength obtained when the oscillation mode of the semiconductor laser module changes from the Bragg grating mode into the Fabry-Perot mode. The de-tuning width xcexdetun is a wavelength range from the shorter-wavelength-side limit value xcexLIMIT of the pulling wavelength width xcexPULL to the gain peak wavelength xcex(Iop).
The optical output and monitor output current of the semiconductor laser can be stabilized by constructing the semiconductor laser module in this manner.
Preferably, the semiconductor laser module is designed so that a threshold current Ith(LD) of the sole semiconductor laser is higher than the threshold current Ith of the modularized semiconductor laser, and an oscillation wavelength xcexLD(Ith) for the threshold current Ith(LD) is shorter than the oscillation wavelength of the modularized semiconductor laser.
FIG. 2A shows gain-wavelength characteristic curves Illustrating optimum relations between the reflection center wavelength xcexBG of the Bragg grating, pulling wavelength width xcexPULL, de-tuning width xcexdetun, shorter-wavelength-side limit value xcexLIMIT of the pulling wavelength width xcexPULL, and gain peak wavelengths xcex(Iop) and xcex(Ith) for the case where an optical output Pf and a monitor output current Im of the semiconductor laser are stable.
On the other hand, FIG. 2B shows gain-wavelength characteristic curves illustrating relations between the reflection center wavelength xcexBG of the Bragg grating, pulling wavelength width xcexPULL, de-tuning width xcexdetun, shorter-wavelength-side limit value xcexLIMIT of the pulling wavelength width xcexPULL, and gain peak wavelengths xcex(Iop) and xcex(Ith) for the case where the monitor output current Im of the semiconductor laser are considerably unstable. In FIG. 2B, {circle around (1+L )} indicates the de-tuning width xcexdetun for the case where a gain peak wavelength xcexLD(Iop1) is greater than a wavelength (=xcexPULLxe2x88x92xcex94xcexBG), where xcex1xcexBG) is the full width at half maximum of the reflection spectrum for the reflection center wavelength xcexBG of the Bragg grating, while {circle around (2+L )} indicates the de-tuning width xcexdetun for the case where a gain peak wavelength xcexLD(Iop2) is greater than the reflection center wavelength xcexBG (=xcexPULL+xcexLIMIT).
With the de-tuning width xcexdetun in the state {circle around (1+L )}, in the semiconductor laser, the oscillation mode competes within the full width at half maximum xcex1xcexBG of the reflection center wavelength xcexBG of the Bragg grating, the monitor output current Im varies at several percent or more, and besides, the time-based change of the oscillation mode is spike-shaped or rectangular.
With the de-tuning width xcexdetun in the state {circle around (2+L )}, moreover, a lot of gain ripple peaks indicated by dotted lines exist near the reflection center wavelength xcexBG of the Bragg grating, and the oscillation mode of the semiconductor laser exists in a region near the reflection center wavelength xcexBG. Thus, the reflection center wavelength xcexBG is situated within the range of a region that is subject to small gain differences, so that the oscillation mode of the semiconductor laser competes between the gain ripple peak wavelengths xcexLD(Iop1) and xcex(Iop2) and the reflection center wavelength xcexBG of the Bragg grating, the monitor output current Im varies at several percent or more, and besides, the time-based change of the oscillation mode is spike-shaped or rectangular.
FIGS. 3A and 3B correspond to the state of FIG. 2B for the case where the monitor output current Im of the semiconductor laser is very unstable. FIG. 3A is a spectrum distribution diagram showing optical outputs in two oscillation states measured in the case where mode competition and temporal mode hopping are caused. FIG. 3B shows an output characteristic curve illustrating a voltage version of time-based change of the monitor output current Im measured in the case where mode hopping is caused.
For comparison, on the other hand, FIGS. 4A and 4B show a spectrum distribution diagram measured when the optical output Pf and the monitor output current Im of the semiconductor laser are in the stable state shown in FIG. 2A and an output characteristic curve illustrating a voltage version of time-based change of the monitor output current Im, respectively. It is to be noted here that the axes of ordinate and abscissa of FIGS. 3B and 4B have different graduations.
Further, the threshold current Ith(LD) of the sole semiconductor laser is higher than the threshold current of the modularized semiconductor laser. In other words, the threshold current Ith of the modularized semiconductor laser is lower than the threshold current Ith(LD) of the sole semiconductor laser (Ith less than Ith(LD)). Thus, lowering of the optical output that is caused as the semiconductor laser is coupled to the optical transmission medium to be modularized by means of the optical coupling means can be compensated by injecting an operating current equal to the threshold current Ith(LD) for the case where the semiconductor laser is used singly.
Thus, lowering of the properties and reliability of the semiconductor laser module of the present invention, which is attributable to the aforesaid catastrophic optical damage (COD) caused by the increase of the optical power density on the semiconductor laser end face, can be restrained, and the range of the injection currents from the threshold current Ith to the maximum operating current Iop can be widened.
Further, the semiconductor laser module of the invention is designed so that the oscillation wavelength xcexLD(Ith) for the threshold current Ith(LD) for the case where the semiconductor laser is used singly is shorter than the oscillation wavelength of the modularized semiconductor laser. Thus, in the semiconductor laser module, the semiconductor laser oscillation state can be stabilized more easily than in the case where the semiconductor laser is used singly. This is because if the respective gain peaks of semiconductor lasers that exhibit gain ripples are compared, sub-peaks on the shorter-wavelength side of a peak gain ripple are higher than sub-peaks on the longer-wavelength side, as seen from FIGS. 1B or 7.
Accordingly, in the semiconductor laser module of the present invention, minor laser oscillation can occur less easily if the reflection center wavelength xcexBG is set on the longer-wavelength side of the peak gain ripple wavelength than in the case where the reflection center wavelength xcexBG is set on the shorter-wavelength side. Thus, in the semiconductor laser module of the present invention, competition between the Bragg grating mode and the Fabry-Perot mode can be eased if the reflection center wavelength xcexBG is set in a wavelength region that is subject to fewer gain ripples without being set in a wavelength region that involves a lot of gain ripples. In other words, in the semiconductor laser module of the present invention, the oscillation mode of the semiconductor laser for the reflection center wavelength xcexBG thus selected is off the other Fabry-Perot mode for easier oscillation, so that the oscillation state of the Bragg grating mode is stable.
Accordingly, in the semiconductor laser module of the present invention, mode competition within the full width at half maximum xcex94xcexBG of the reflection center wavelength xcexBG of the Bragg grating, which makes the optical output Pf and the monitor output current Im of the semiconductor laser unstable, cannot easily occur, so that the competition between the Bragg grating mode and the Fabry-Perot mode can be eased effectively.
Preferably, the semiconductor laser module is designed so that the reflection center wavelength xcexBG of the Bragg grating is set on the longer-wavelength side of the gain peak wavelength xcex(Iop) of the semiconductor laser for at least one gain ripple (e.g., 3 nm longer-wavelength side in FIG. 1B) shown in FIG. 1B. Thus, in the semiconductor laser module, the optical output Pf and the monitor output current Im of the semiconductor laser can be stabilized more securely.
A semiconductor laser that has ripples in its gain-wavelength characteristic may possibly oscillates even with a sub-peak wavelength that has a gain near that of a main gain peak wavelength and is situated on the longer-wavelength side for one gain ripple. If the Fabry-Perot mode is generated with this sub-peak wavelength without the aforesaid setting, in the semiconductor laser of this type, it competes with the Bragg grating mode that is generated within the full width at half maximum xcex94xcexBG of the reflection center wavelength xcexBG, so that the optical output Pf and the monitor output current Im are subject to fluctuation.
Preferably, moreover, the semiconductor laser module is designed so that the gain ripple interval of the semiconductor laser is not longer than 3.5 nm. It undergoes mode hopping, if any, of 3.5 nm or less. Thus, in the semiconductor laser module, the pulling wavelength width need not be enlarged unduly.
In other words, although the pulling wavelength width in the semiconductor laser module can be enlarged by increasing the reflectance of the Bragg grating, the optical output inevitably lowers if the reflectance increases. On the other hand, the semiconductor laser module can be designed so that the pulling wavelength width can be enlarged by lowering the reflectance of an anti-reflection film with which the front end face of the semiconductor laser is coated. In consideration of manufacturing processes, however, it is hard to lower the reflectance of the anti-reflection film to 1% or less. Further, the semiconductor laser module can be designed so that the pulling wavelength width can be enlarged by enhancing the efficiency of coupling between the optical transmission medium and the semiconductor laser. Since the longitudinal and transverse mode fields of the semiconductor laser are different from the mode fields of the optical transmission medium, however, it is hard to obtain a high efficiency of coupling between the semiconductor laser and the optical transmission medium.
Preferably, furthermore, the semiconductor laser module is designed so that the difference between the reflection center wavelength xcexBG of the Bragg grating and the gain peak wavelength xcex(Ith) is set at a large value given by [xcexBGxe2x88x92xcexLD(Ith)] greater than xcex9sxc3x97(Iopxe2x88x92Ith), with respect to the product of a shift ratio xcex9s (nm/mA) of the gain peak wavelength for each injection current in the semiconductor laser to a shift on the longer-wavelength side and the difference between the maximum operating current Iop and the threshold current Ith. Thus, the semiconductor laser module is designed so that the characteristics of the semiconductor laser to be used can be easily noticed according to data on the semiconductor laser.
This is because the shift ratio xcex9s based on the injection current of the completed semiconductor laser can be controlled substantially with good reproducibility to fulfill the specifications in the semiconductor laser manufacturing processes, according to experiences and accumulated manufacturing data on production lots. In the semiconductor laser, moreover, the value of xcex9sxc3x97(Iopxe2x88x92Ith) is controlled at all times. In the semiconductor laser module, therefore, the semiconductor laser to be used can be easily selected if the gain peak wavelength xcex(Ith) of the threshold current Ith of semiconductor laser and the reflection center wavelength xcexBG of the Bragg grating are known.
Preferably, the semiconductor laser module is designed so that the difference between the reflection center wavelength xcexBG of the Bragg grating and the gain peak wavelength xcex(Ith) is set at 7 nm or more. Thus, the semiconductor laser module can use a conventional GaAs/AlGaAs-based semiconductor laser that has ripples in its gain-wavelength characteristic.
In general, the semiconductor laser has a band-filling effect such that the gain peak wavelength xcex(Ith) shifts on the shorter-wavelength side if the injection current is increased and a temperature increase effect for a shift on the longer-wavelength side. It is known that the band-filling effect develops remarkably on the lower-current side, and the temperature increase effect on the higher-current side. These two effects are intricately linked together. According to experience, the average of shift ratios xcex9s for the case where the gain peak wavelength xcex(Ith) shifts on the longer-wavelength side as the injection current increases ranges from 0.02 to 0.03 (nm/mA).
Thus, in the case where a 980 nm band semiconductor laser having an output of, for example, 100 mW or thereabout is used singly, the increase of an injection current If from 40 mA to 240 mA with the oscillation threshold current at about 40 mA is 200 mA, and the shift of the oscillation wavelength based on the injection current If is about 5 nm (=976xe2x88x92971 nm), as shown in FIG. 5. In consideration of ripples in the semiconductor laser module also, therefore, it is evident that the reflection center wavelength xcexBG should be selected so that the difference (=xcexBGxe2x88x92xcexLD(Ith)) between the reflection center wavelength xcexBG and the gain peak wavelength xcex(Ith) is 7 nm or more. FIG. 5 will be described further in detail later in connection with an embodiment of the present invention.
More specifically, the semiconductor laser module set in this manner is designed taking notice of the wavelength characteristic of the optical output so that 60% or more of the spectrum component of the optical output power of the semiconductor laser module is concentrated within the xc2x11 full width at half maximum of the reflection center wavelength xcexBG of the Bragg grating, and that less than 40% of the spectrum component is within the xc2x11 gain ripple interval of a gain peak wavelength xcexLD(I) for an injection current I for the case where the semiconductor laser is used singly. Thus, in the semiconductor laser module having this power distribution, both the optical output Pf and the monitor output current Im can be stabilized.
Preferably, in this case, the power ratio between optical output power for the reflection center wavelength xcexBG and optical output power for the gain peak wavelength xcexLD(I) is 10 dB or more. In the semiconductor laser module, this power ratio can be utilized as a criterion for screening.
Preferably, moreover, the full width at half maximum of the gain spectrum of the semiconductor laser is 15 nm or more.
Preferably, furthermore, the absorption coefficient of the semiconductor laser is 15 cmxe2x88x921 or less.
Preferably, the efficiency of coupling between the semiconductor laser and the optical transmission medium by means of the optical coupling means is 60% or more. Thus, the semiconductor laser module is designed so that lowering of the optical output that is attributable to the connection of the optical transmission medium having the Bragg grating to the semiconductor laser can be restrained, and minimum requirements for the pulling wavelength width can be fulfilled.
The optical coupling means that is used to couple the optical transmission medium and the semiconductor laser may be either a wedge-lensed fiber or an asymmetrical two-lens system.
Preferably, moreover, the front end face of the semiconductor laser is coated with an anti-reflection film having a reflectance of 6% or less. In this case, the semiconductor laser module can enjoy stable oscillation characteristics within a narrow wavelength region if the pulling wavelength width xcexPULL, which is settled depending on parameters such as the reflectance of the anti-reflection film, the gain spectrum full width at half maximum, absorption coefficient, feedback coupling efficiency of the semiconductor laser, and the reflectance for the reflection center wavelength xcexBG of the Bragg grating, is 4 nm or more.
Preferably, furthermore, the semiconductor laser is provided with temperature control means such that a desired gain peak wavelength xcexLD(I) can be outputted for a given injection current I. For example, a Peltier device may be used as the temperature control means. The control temperature is adjusted to room temperature or the working temperature of the semiconductor laser. In the case of a 980 nm band semiconductor laser that is used in an ordinary erbium doped fiber amplifier (EDFA), for example, the control temperature is adjusted to 25xc2x0 C.
Change of the control temperature is equivalent to change of the gain peak wavelength of the semiconductor laser. Naturally, the reflection center wavelength xcexBG of the Bragg grating and the gain peak wavelength are set at a fixed temperature as a precondition.
It is known that the oscillation wavelength of the semiconductor laser is changed by changing the temperature. This is done because the gain peak wavelength of the semiconductor laser changes depending on a temperature Ta or Tc. The temperature Ta is the active layer temperature of the semiconductor laser, while the temperature Tc is the package temperature of the semiconductor laser.
In the case of the semiconductor laser to which the optical transmission medium having the Bragg grating is coupled, however, the oscillation wavelength is locked by the reflection center wavelength xcexBG of the Bragg grating if the gain peak wavelength changes. As long as the difference between the gain peak wavelength and the reflection center wavelength xcexBG is within the range of the pulling wavelength width xcexPULL, the oscillation wavelength never fails be situated corresponding to the reflection center wavelength xcexBG. If the relation between and preset conditions for the reflection center wavelength xcexBG and the gain peak wavelength of the semiconductor laser module are not fulfilled, the optical output and the monitor output current inevitably fluctuate to a marked degree.
If the semiconductor laser in the semiconductor laser module is expected to be oscillated within the range of a given wavelength, therefore, the oscillation wavelength set according to the reflection center wavelength of the Bragg grating of the optical transmission medium, and the gain peak wavelength is controlled by controlling the temperature of the semiconductor laser by means of the temperature control means. By doing this, the relation between and the preset conditions for the reflection center wavelength xcexBG and the gain peak wavelength of the semiconductor laser module can be fulfilled, and the selection of the semiconductor laser can be favorably facilitated further. Thus, in the semiconductor laser module of the present invention, it is also advisable to control the gain peak wavelength of the semiconductor laser according to the temperature and set the oscillation wavelength according to the reflection center wavelength xcexBG.
Preferably, the reflectance of the optical transmission medium for the reflection center wavelength xcexBG of the Bragg grating is 3% or more.